1. Field of the Invention
The present invention relates to an image forming device, such as a display using an electron beam and, more specifically, to an image forming device including spacers.
2. Description of the Related Art
A known image forming device using an electron emitter is a flat display panel. The known flat display panel comprises an electron source substrate including a plurality of cold cathode electron emitters and an anode substrate including an anode electrode and phosphors. The electron source substrate and anode substrate are disposed parallel to each other. A vacuum is generated between the electron source substrate and the anode substrate. Generally known cold cathode electron emitters are surface-conduction type emitters, field electron emission (FE) type emitters, and metal-insulator-metal (MIM) type emitters. The flat display panel including known cold cathode electron emitters is light-weight and has a large display area compared to other widely used CRTs. Moreover, the flat display panel is brighter and is capable of displaying higher quality images compared to other flat display panels using liquid crystal and flat display panels such as plasma displays and electroluminescent displays.
In general, the above-described image forming device comprises a face plate and a rear plate facing each other. The face plate is the display surface for displaying an image. The face plate includes a metal back, which receives an acceleration voltage Va, and a fluorescent film. The rear plate is the electron source for generating light from the phosphors. The rear plate includes cold cathode electron emitters and wires, wherein the wires electrically connect the electron emitters and run in the longitudinal and horizontal directions. Sidewalls seal the circumference of the face plate and the rear plate, forming a vacuum vessel. Spacers are interposed between the face plate and the rear plate to maintain the plates apart from each other at a predetermined distance and to support the plates against atmospheric pressure. The spacers are usually interposed between and are in contact with the conductor of the rear plate (e.g., the wires in the horizontal direction) and the electrode on the face plate (e.g., the metal back) (for example, refer to U.S. Pat. Nos. 5,614,781 and 5,742,117 and Japanese Patent Laid-Open No. 08-180821).
In such an image forming device, sometimes the spacers emit a secondary electron when a part of an electron beam or a reflected electron strikes the surface. This secondary electron generates an electric potential in the area where the secondary electron was emitted from. Accordingly, the electric potential distribution at the spacer and the vicinity is distorted. As a result, not only the trajectory of the electron beam becomes unstable but also an electric discharge will occur inside the image forming device.
To prevent electrical charging of the spacers, the spacers may be formed of an insulating substrate covered with a high-resistance film, which is capable of preventing electrical charging. This method of preventing electrical charging is disclosed in, for example, U.S. Pat. Nos. 5,614,781 and 5,742,117 and Japanese Patent Laid-Open No. 08-180821.
The inventors propose a more preferable method for preventing electrical charging of spacers in which spacers formed of an insulating substrate covered with a high-resistance film are disposed intermittently in contact with the conductors on the rear plate (refer to Japanese Patent Application No. 2003-136741).
However, when the contact area of the spacer actually in contact with the conductor is small in comparison with the surface area (including the contact area) that faces the conductor, electrical currents are converged (or, in other words, current crowding occurs) at the edge of the contact area. This current crowding occurs, for example, when the spaces contact the conductors intermittently, as described above, or when the thickness (width) of the planer spacers is greater than the width of the conductors in contact.
FIG. 11 illustrates the latter case in which the contact area of a spacer 1020 contacting a conductor (horizontal wire 1013) on a rear plate 1015 or an electrode (metal back 1019) on a face plate 1017 is smaller than the area of the surface including the contact area. In such a case, current crowding occurs at the edges of the contact area (points b in the drawing). Due to current crowding, heat is generated locally at the points b and the vicinity. Therefore, depending on the type of material used for a high-resistance film 1001, the property of the film (such as resistance) may change when the high-resistance film 1001 is used for a long period of time (i.e., when Va is applied for a long period of time). As a result, the electric field in the vicinity of the spacer 1020 is distorted, causing the formed images to be distorted. FIG. 11 also illustrates an insulating substrate 1000, a fluorescent film 1018, a longitudinal wire 1014, and an insulating layer 1021.
Current crowding that occurs at some of the edges of the high-resistance film even when the high-resistance film is disposed on the edge of the spacer, as illustrated in FIG. 11, is known to be caused by the relationship of electric properties between the high-resistance film on the side of the spacer, the film on the edge of the spacer, and the conductor in contact with the spacer in addition to the above-described case in which the contact area of the spacer is only partially in contact with the rear plate or the face plate contact. When the entire end surface of the spacer is a contact area, it is desired to effectively use the entire high-resistance film on the end surface as a current path.